Temperature dependence of fluorescence and phosphorescence of the triphenylamine dimer 3-methyl-TPD

Abstract

The triphenylamine dimer, N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD) as neat film, and as guest in polystyrene (PS), 4,4′-N,N′-dicarbazole-biphenyl (CBP), and polycarbonate (PC) films is investigated by luminescence spectroscopy. The luminescence quantum distributions, E_L(λ), are determined over a wide temperature range from 10 K to 291 K by photo-excitation in the singlet absorption region of TPD. The singlet-excited phosphorescence quantum distributions are extracted by fluorescence contribution subtraction. The absolute intrinsic luminescence quantum yields, fluorescence quantum yields, and singlet-excited phosphorescence quantum yields are calculated from the quantum distributions. Even at room temperature phosphorescence emission is obtained. The triplet-singlet radiative lifetimes are determined from extracted S₀–T₁ absorption and emission spectra, and the phosphorescence lifetimes are estimated from the phosphorescence quantum yields and the radiative phosphorescence lifetimes. In TPD neat films fluorescence reduction by self-quenching is observed. CBP host to TPD guest excitation transfer is observed in the case of excitation into the CBP absorption region.

Introduction

Thin films of the aromatic diamine molecule N,N′-diphenyl-bis(3-methylphenyl)-biphenyl-4,4′-diamine (also named N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine, abbreviated by TPD or more precisely by 3-methyl-TPD) are widely used as hole-transport layers (HTL) in organic light emitting diodes (OLEDs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. TPD is a laser active material. Whispering-gallery-mode lasing of a TPD coated optical fibre was reported [10]. Travelling-wave lasing of TPD solutions and neat films was achieved [11]. Laser action of TPD based conjugated and non-conjugated polymers was achieved [12], [13], [14].

4-Methyl-TPD (N,N′-bis(4-methylphenyl)-N,N′-diphenyl-benzidine) also works as thin film laser material [15]. Laser action of a copolymer of 4-methyl-TPD with phenyl substituted MEH-PPV was achieved [15]. Recently laser action was shown on un-substituted TPD (N,N,N′,N′-tetraphenylbenzidine) [16].

TPD is used as host material for phosphorescent guest molecules like PtOEP and Ir(ppy)₃ [17], [18]. In the case of excitation into the TPD host absorption region efficient Förster-type energy transfer [19] from TPD to Ir(ppy)₃ and PtOEP was observed [17], [18], [20]. Dexter-type [21] triplet–triplet forth and back excitation transfer between TPD and Ir(ppy)₃ and triplet–triplet forward transfer from TPD to PtOEP were revealed [17], [18], [20]. A numerical simulation of the temperature dependence of the photoluminescence of Ir(ppy)₃ in TPD due to triplet–triplet Dexter-type energy transfer is given in [22].

In TPD doped thin polystyrene films efficient Förster type energy transfer from singlet excited polystyrene to TPD was found when the excitation wavelength was in the polystyrene absorption wavelength region [22].

In this paper, the luminescence behaviour of a neat thin film of TPD and of thin films of 5 wt-% TPD in the hosts polystyrene (PS), 4,4′-N,N′-dicarbazole-biphenyl (CBP), and bisphenol-A-polycarbonate (PC) is studied over a wide temperature range from about 10 K to room temperature. The structural formulae of TPD and the hosts PS, CBP, and PC are shown in Fig. 1. The absorption cross-section spectra of TPD and of the hosts are shown in Fig. 2.

Polystyrene is used in OLEDs fabrication as host polymer for forming high quality thin films with good mechanical properties [20], [23], [24], [25]. The absorption and fluorescence behaviour of polystyrene is investigated in [20]. CBP is used in OLEDs as host material for triplet emitters [26], [27]. The fluorescence spectrum and phosphorescence spectrum of CBP are found in [27], [28]. Polycarbonate (bisphenol-A-polycarbonate) is an important plastic material known for its excellent transparency and high impact resistance [29]. It is applied as OLED substrate [30]. The fluorescence behaviour of bisphenol-A-polycarbonate is studied in [29].

The TPD neat films and the TPD doped films were prepared by spin-coating. The temperature dependent luminescence behaviour is compared between untreated films (stored in the dark under ambient conditions till measurement) and annealed films (stored in the dark under ambient conditions and heated to 80 °C for 1 h before measurement). The luminescence spectra are composed of fluorescence contributions and phosphorescence contributions. These contributions are separated out in an analysis of the luminescence spectra. The radiative phosphorescence lifetimes are calculated by applying the Strickler–Berg formalism to the singlet–triplet absorption and phosphorescence spectra. Phosphorescence lifetimes are then estimated from the radiative phosphorescence lifetimes and the phosphorescence quantum yields.

The intention of this paper is to determine absolute fluorescence quantum yields and absolute phosphorescence quantum yields of a 3-methyl-TPD in neat film and of 3-methyl-TPD doped into polymeric hosts (polystyrene, polycarbonate) and in a molecular host (CBP) as a function of temperature. A luminescence analysis method is developed to separate out approximately the fluorescence quantum distributions and the phosphorescence quantum distributions from quantitative luminescence quantum distribution measurements. Phosphorescence lifetimes are estimated from singlet–triplet absorption data and phosphorescence spectra by applying the Einstein relations between absorption, stimulated emission and spontaneous emission [31], [32]. A neat film of TPD was studied to get information about possibly temperature dependent luminescence self-quenching. For the doped films a concentration of 5 wt-% TPD was selected as a typical doping level used in OLEDs [27]. For TPD in polystyrene four samples with molar mass spanning the range range M_w = 2000–500 000 g mol⁻¹ were investigated since a first studied untreated film with M_w = 2000 g mol⁻¹ (containing some residual tetrahydrofuran solvent from preparation) exhibited an unexpected luminescence discontinuity at a temperature of about 150 K, and the reason for this discontinuity was intended to find out. An excitation transfer from host to guest in the case of excitation into the shorter wavelength absorption region of the host is monitored.